Photoelectric Effect
Although obscure to most people who aren't scientists and students,
the photoelectric effect is nonetheless an important phenomenon
to know about. So much so that, in 1921, Albert Einstein received
the Nobel Prize in Physics for his explanation of it, and not for
the theory of relativity. What is it? In general, the photoelectric
effect is whenever electromagnetic radiation is absorbed by a material,
causing it to release charged particles. In practice, this is usually
observed with ultraviolet light and metals, which release electrons
easily.
The photoelectric effect has some curious properties that cannot
be explained by classical physics. It was found that the number
of electrons released by a metal was proportional to the amount
of ultraviolet light, while the energy of the electrons depended
on the frequency of the light. Below a threshold frequency, there
are no electrons released at all, no matter how bright the light
is, while above the threshold frequency, there are always electrons
released, no matter how dim the light is. This contradicted classical
physics because classical wave theory stated that either increased
intensity or increased frequency should provide more energy in the
same way, but the observed effect showed that only increased frequency
provided the needed energy to eject an electron from the metal.
The implications turned out to be far-reaching. These findings
were finally explained by Einstein in this Ph.D. thesis in 1905:
he proposed that light could not only be waves, but could also come
in packets of energy, as photons. As a photon hit an electron, it
would provide it with a certain amount of energy. If it was enough,
the electron would be kicked to the surface of the metal and be
observed, but if it wasn't enough, the electron would fall back
to its atom. Therefore, photons with energy below the threshold
had no discernible effect and the number of photons could only determine
how many electrons were released, as only one photon at a time was
likely to hit an electron.
The photoelectric effect was indisputable evidence of photons
and thus, it kicked off the quantum
physics revolution.
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